1. Field of the Invention
The present invention relates to a radio communication system, in particular to a method and an apparatus for providing mutual time difference determination of base station signals in a cellular communication system.
2. Description of the Related Art
Third Generation (3G) CDMA cellular base stations are non-synchronous in for example the Third Generation Partnership Project (3GPP) Frequency Division Duplex (FDD) mode. Here, the synchronism of base stations (BS) refers to the synchronization of the downlink transmission signals of different base stations.
Mutual time difference determination is required to determine the mobile users' locations and to reduce time and hardware resources at the initial base station signal search by mobile stations and to decrease the stored data amount in the course of the soft handoff procedure. The mutual time difference determination during the soft handoff procedure is described in the Russian patent #2137314, Int. cl.6 H 04 L 27/30, which is incorporated herein by reference.
One of the prior art solutions described in “Synergies Between Satellite Navigation and Location Services of Terrestrial Mobile Communication”, G. Hein, B. Eissfeller, V. Oehler, Jon O. Winkel, Institute of Geodesy and Navigation, University FAF Munich, ION GPS 2000, 19-22 Sep. 2000, Salt Lake City, Utah, which is incorporated herein by reference proposes the application of location measurement units receiving BS signals and defining the BS their mutual time difference to determine mutual time differences of base stations.
However, the disclosed solution described above has the following problems:
First, non-line of sight multipath signal propagation from base stations to location measurement units causes low accuracy when determining mutual time differences of base station signals.
Second, it is impossible to get the mutual time difference of the base station signals when the direct measurement is unobtainable.
The prior art solution to overcome such problems is the solution described in 3GPP Technical Specification (TS) 25.305 V3.7.0, 2001 12, “Stage 2 Functional Specification of UE Positioning in UTRAN”, which is incorporated herein by reference.
This solution proposes to receive base station signals, from which the mutual time difference is determined by the location measurement unit which is located in a position with known coordinates. FIG. 1 helps in understanding the method of mutual time difference determination of base station signals in a cellular communication system performed by 3GPP TS 25.305 V3.7.0, 2001 12.
FIG. 1 shows base stations 1 and 2, the mutual time difference of which is to be determined, a location measurement unit (LMU) 3, base station controller (BSC) 4 and a mobile user location center (MULC) 5.
The LMU 3 receives a signal of the base station (BS) 1 and a signal of the BS 2 and executes a specified number of sequential measurements of the signals' time difference. Further, the LMU 3 averages the sequential measurements of time difference of BS 1 and BS 2 signals, thus obtaining the averaged measured time difference of the signals of the given base stations.
The LMU 3 determines the accuracy of the averaged measured time difference, for example, by the signal-to-noise ratio of the BS 1 and BS 2. In this case a value linearly connected with an error of the averaged measured time difference to the true value of the time difference of the BS 1 and 2 signals received by the location measurement unit 3 is selected as the averaged measured time difference accuracy.
The averaged measured time difference of the BS 1 and BS 2 signals and its accuracy are transmitted from the LMU 3 to the BS 1 or BS 2 using the current radio interface and then transmitted from the respective BS to the base station controller 4 by a wire communication line connecting the BSC 4 and the base station.
The BSC 4 determines the mutual time difference of the BS 1 and BS 2 signals by the averaged measured time difference of the BS 1 and BS 2 signals by considering the known mutual location of the BS 1 and BS 2 and the LMU 3.
The obtained mutual time difference of the BS 1 and BS 2 signals and its accuracy are transmitted from the BSC 4 to the mobile user location center 5 for further application.
Thus, the following main features of implementing the method can be emphasized from the description of the prior art method of mutual time difference determination of base station signals in a cellular communication system:
In each location, a measurement unit performs the sequential measurement of mutual time difference of signals of at least two base stations, signals of which are received in the location measurement unit, averaging of these time difference measurements results in the averaged measured mutual time difference of these base stations, and determines its accuracy.
The averaged measured time differences and their accuracies are transmitted from each location measurement unit to one of the base stations which signals are received by the location measurement unit, and then to the base station controller that controls the base station.
The mutual time difference of signals is determined for each base station pair for the base station controller using the averaged measured time difference of the given base station signals.
However, the known method has a number of important disadvantages.
First, determination of the mutual time difference of the base station signals by the averaged measured time difference of these base station signals may be insignificantly accurate. The inaccuracy is caused by the fact that the estimate of the mutual time difference of the base station signals is subject to the impact of noise errors, intra-system interference and multipath errors.
The difference between the delays of line of sight base station signals propagation to the location measurement unit can be removed with the use of the known coordinates of the base stations and location measurement unit.
When Δt1−>2 is marked as an estimate of the mutual time difference of signals of the base station pair, especially the signals of the first BS to the signal of the second BS, and Δ{tilde over (t)}1−>2 is marked as the true value of the mutual time difference of signals of the base stations, the difference between the estimate Δt1−>2 and the true value Δ{tilde over (t)}1−>2 can be expressed by the following Equation 1.Δt1−>2−Δ{tilde over (t)}1−>2=εnoise+εmultipath,1−εmultipath,2  Equation 1
In equation 1, εnoise is an error defined by noise and intra-system interference, εmultipath,1 is a multipath error of the first BS, equal to the difference between the real propagation time of the first BS signal to the location measurement unit and the known propagation time of the first BS signal to the location measurement unit, and εmultipath,2 is a multipath error of the second BS, equal to the difference between the real propagation time of the second BS signal to the location measurement unit and the known propagation time of the second BS signal to the location measurement unit.
The averaging of the estimate Δt1−>2 of the mutual time difference of signals of the first BS and second BS in the location measurement unit results in the reduction of the noise error value εnoise. The multipath error difference εmultipath,1−εmultipath,2 is invariable since it is defined by the mutual location of the first and second BS and the location measurement unit and the surrounding scattering objects (buildings, mountains, hills, etc.) as well.
Hence, the accuracy of the claimed method of mutual time difference determination of base station signals in a cellular communication system may be insufficient for a location.
Second, there may be a situation when there is no direct time signal difference measurement between any base stations, even when their mutual time difference is required.
FIG. 2 illustrates the above situation, demonstrating the base stations 6, 7 and 8, location measurement units 9 and 10 and a building 11. Each of the BS 6, BS 7 and BS 8 transmits the first and second signals which comprise a group signal.
The base station group signal refers to a signal transmitted from the base station and having the Synchronization CHannel (SCH), Common PIlot Channel (CPICH) and Primary Common Control Physical CHannel (P-CCPCH) and other channels as well.
Each of the LMU 9 and the LMU 10 receives signals transmitted from surrounding base stations and measures the signals time difference.
Referring to FIG. 2, the LMU 9 receives the first signals of the BS 6 and BS 7 and measures their time difference. The LMU 10 receives the second signal of the BS 7 and the first signal of the BS 8 and measures their time difference.
However, the second signal of the BS 6 is held by the building 11 and cannot be received by the LMU 10. The second signal of the BS 8 is also held by the building 11 and cannot be received by the LMU 9.
When the direct measurement of the time difference of the signals of the BS 6 and BS 8 cannot be performed as described above, the conventional methods do not propose any solution capable of determining the mutual time difference between the signals of the BS 6 and BS 8.